Inductive power transfer (IPT) systems are known and used in a number of applications. IPT technology offers high efficiency, typically 85-90%, and is capable of operating in hostile environments, being unaffected by dirt and moisture.
A typical IPT system consists of three main components: an AC power supply; a primary conductive path, coil, or pad electrically coupled to the power supply; and at least one electrically isolated pick-up which, in use, is inductively coupled with the primary winding to supply power to a load. The power supply and primary winding are commonly said to comprise the primary side of the IPT system, and the pick-up(s) and associated load(s) comprise the secondary side of the system.
Traditionally, the power supply would be electrically coupled to an electrical supply network (commonly referred to as the “grid”) and used to energise the primary winding for contactless uni-directional power transfer from the grid to the load(s). The primary winding is energised by a converter, derived from the AC or grid power supply, to create a high frequency AC current in the primary winding, which in turn results in generating a continuously varying magnetic field about the primary winding. The or each pick-up, separated from the primary winding by an air gap, includes a coil in which a voltage is induced by the changing magnetic flux passing through the coil in accordance with Faraday's law of induction, thereby achieving contactless power transfer.
Generally a converter, serving as an inverter, on the primary side and a simple switch mode regulator on the pick-up side are adequate to control the power flow in uni-directional IPT systems, as only the primary side sources power.
Bi-directional IPT systems are ideal for applications such as electric vehicles (EVs) and vehicle-to-grid (V2G) systems, for example, in which ‘contactless’ two way power transfer is desirable, for the purpose of balancing the load on the grid.
However, the power flow of bi-directional IPT systems must be regulated using a more sophisticated control strategy in comparison to uni-directional systems.
In contrast to a uni-directional IPT system, both the primary and pick-up sides of a bi-directional IPT system must be configured to serve as either a source or a sink. Consequently, identical or similar converter topologies are required on both sides of the system to facilitate either AC-DC or DC-AC power conversion, depending on the direction of power flow.
To minimize the VA ratings of converters for any given power level, parallel or series compensation is usually provided for coil inductances of both the primary and pick-up. Thus bi-directional IPT systems are higher order resonant networks, and require more sophisticated and robust control.
The amount and direction of power flow are usually controlled by either relative phase or magnitude control of the voltages produced by converters, using dedicated controllers. However, a robust mechanism is essential to ensure that the power demand from one side can be met by the supplying side without exceeding its power rating.
Converters employed in parallel compensated IPT systems naturally behave as a current source. In such systems, limiting the operation of the sourcing side to its maximum or rated power level has always been a challenge, especially when the pick-up demand exceeds the power handling capability of the sourcing converter. Prime examples occur when the magnetic coupling between the two sides improves or pick-ups with varying power demands are to be catered for by a single primary converter as in the case of electric vehicles (EVs) charged at a charging bay. Moreover, sharing and prioritizing of power delivery among multiple pick-ups can also be regarded as a challenge for any given input power.
Power supplied by the primary can be limited by lowering the primary coil or track current upon detection of an excessive power demand. But this strategy will not suit multiple pick-up IPT systems as it compromises the power delivery to other pick-ups. Moreover, single pick-up IPT systems that use lower track currents operate at relatively high quality factors, leading to more circulating energy and making the system more susceptible to component tolerances.
An alternative solution is to employ a dedicated wireless communication interface between the primary side and the or each pick-up. The maximum power capability of the primary can then be relayed to pick-ups through the wireless interface, requesting that the power intake should be limited. However, such a wireless interface would obviously be expensive in terms of component count and complexity as it requires additional hardware and software.